1. Field of the Invention
The present invention is directed to granular compositions containing insecticidal phosphoroamidothioates as well as to methods for employing said compositions. In particular, the present invention is directed to chemically stable granules of insecticidal phosphoroamidothioates which granules contain granular processing aids and a sufficient amount of ammonium sulfate so as to impart chemical stability to the phosphoroamidothioates. The present invention is also directed to methods for killing insects which utilize such compositions.
2. State of the Art
Insecticidal compositions in various forms are available for rather diverse methods of application. The method of preparation of the insecticidal composition is largely determined by the physical and chemical nature of the insecticide and the intended use and method of application of the insecticide to the area to be treated.
Certain phosphoroamidothioates and phosphoroamidodithioates ("phosphoroamidothioates") are known in the art as having excellent insecticidal activity against a variety of insects and in a variety of environments. A particularly important commercial insecticide within these classes of compounds is the insecticide acephate (generic name) or Orthene.RTM. (tradename) which can be systemically taken up by a plant so that insects which feed and/or live on the plant are killed, in addition to those insects which directly ingest or are contacted by the insecticide. Acephate and related compounds are described in U.S. Pat. Nos. 3,716,600, 3,845,172 and 3,914,417, which references disclose that in addition to their insecticidal properties, the compounds disclosed therein possess very low mammalian toxicity. Orthene.RTM. is commercially produced as a technical grade chemical of about 97 to 99.5% purity.
One method of formulating technical grade phosphoroamidothioates for commercial use is to mix the technical grade powder with an anti-caking agent, such as fumed silica, and a wetting agent. The wetting agent is utilized to wet the insecticide and the anti-caking agent is used to prevent agglomeration of the insecticide in its container. This formulation of insecticide can be applied to crops as a spray solution or as a dust.
Use of phosphoroamidothioates as powders allows for relatively high concentrations of insecticide to be applied to a treatment area, but the powder application suffers from various disadvantages. First, the finely divided particles of active spray may be carried by air currents into areas where harmful effects may occur. In addition, it is difficult to apply sprays or dusts to the soil surface or to lower areas of plants when dense foliage must be penetrated. Finally, powdered phosphoroamidothioates suffer from chemical stability problems due to hydrolytic and catalytic driven degradation which shortens the shelf life of the powdered insecticide.
While the use of granules (e.g., pellets) would overcome some of the inherent difficulties involved in using sprays or dusts, compression formed granular formulations are generally prepared using processing aids such as lubricants, flowability agents, etc. which, in the present invention have been determined to aggravate the chemical stability problems of insecticidal phosphoroamidothioates. In turn, the lack of chemical stability for granular insecticidal phosphoroamidothioates interferes with the commercial feasibility of such formulations.
In the present invention, it has been found that granules of insecticidal phosphoroamidothioates containing processing aids can be made chemically stable by the use of a sufficient amount of ammonium sulfate. Specifically, in the present invention, granules of insecticidal phosphoroamidothioates containing less than about 6 weight percent of processing aids and at least about 75 weight percent of ammonium sulfate possess improved chemical stability as compared to similar granules containing less than 75 weight percent ammonium sulfate. Contrarily, substitution of other commonly employed carrier materials (e.g., bentonite, diatomaceous earth, attapulgite, etc.) for ammonium sulfate in the insecticidal phosphoroamidothioate granules do not impart chemical stability to the granule and, in fact, appear to compound the instability problem.
While the use of minor amounts of ammonium sulfate with phosphoroamidothioates in pellets (granules) is taught in PCT Application No. PCT/US91/11104 (less than 5 weight percent ammonium sulfate); in European Patent Application No. 415,609 (less than 5 weight percent ammonium sulfate); and in U.S. Ser. No. 07/491,497 filed Mar. 16, 1990 and to be issued as U.S. Pat. No. 5,100,667, on Mar. 31, 1992 (less than 40 weight percent ammonium sulfate), the use of at least 75 weight percent ammonium sulfate has not been taught or suggested in these references nor has it been suggested that the use of such amounts would impart improved chemical stability to granules containing insecticidal phosphoroamidothioates as compared to use of less than 75 weight percent.
Likewise, while U.S. Pat. No. 3,585,022, discloses granules containing a pesticide and from 30 to 90 parts by weight of crystalline salt, this reference fails to teach the use of ammonium sulfate or granules of ammonium sulfate in combination with insecticidal phosphoroamidothioates. Moreover, this reference fails to disclose that use of a sufficient amount of ammonium sulfate will impart chemical stability to the phosphoroamidothioates.